The invention concerns turbine housings for jet engines and more particularly rings placed at the periphery of turbine wheels and constituting part of the housing, said rings operating at elevated temperatures and being exposed to the abrasion caused by the tips of the rotating blades.
In a turboreactor, the value of the thrust produced is proportional both to the amount of air and the velocity of flow of the exhaust gas. The air flow originating in the compressor and the combustion chamber is directed onto the turbine blades mounted on rotating disks. The kinetic energy of the jet of gas is transformed by its passage through the blades into a rotational couple which moves the disks. The disks of the turbine are integral with a shaft connected with the inlet compressor. Because the output of the engine is due, in part, to the transfer of the kinetic energy of the flow of air to the turbine, it is important that all of the air should pass through the blades by eliminating leaks developing between the tips of the blades and the housing which surrounds them and which serves to outline the flow section. In view of the movement of the pieces and the elevated operating temperatures, it has not been possible to eliminate in a simple manner the space between the tips of the blades and the housing.
In order to reduce this space, if not completely eliminate it, numerous solutions have been proposed. According to a rather old solution, the housing was doubled by rings arranged to face each disk of blades. Because of the impossibility of maintaining the small tolerances between the tipes of the blades and the rings, a wear problem appeared, which was aggravated still more by the deformations produced by the elevated temperatures at which these parts operate. This leads to the cooling of the rings. In order to obtain an effective joint between the tip of the blade and the ring without wearing off the tip of the blades, a material wearing more readily than the material constituting the blades was used in the rings, the blades then forming a groove in the ring in which they rotate freely.
The initial honeycomb materials used in the rings allowed only impact cooling which was often inadequate and not very uniform. Transpiration cooling, i.e. by traversing the material with a coolant, can be used only with a low porosity material, because the amount of the cooling liquid must be kept as low as possible to avoid a reduction of the overall output of the reactor.
The need for low porosity lead to the making of the rings of porous sintered materials. The abrasion produced by the blades had as a corollary an important and uncontrollable variation of porosity so that cooling by transpiration could not be effected correctly.
U.S. Pat. No. 3,825,364 proposed a solution to this problem by using an abradable material consisting of two layers of sintered metal of different porosities. The material controlling the flow of the cooling liquid is the outer one with respect to the blades, while the material in contact with the blades has a high porosity so that it may be worn down by the blades without interferring with the passage of the cooling liquid toward the outside. The cooling of the ring alone cannot provide completely satisfactory results if its support itself is subject to thermal deformation. The housing and the device supporting the ring are cooled by a flow of air uniformly distributed through the openings of a jacket arranged at a small distance from the walls of the housing. The ring, made of the sintered material, is held in a support which leaves a space between the bottom of the support and the ring, said space being supplied with cooling air from the jacket, the air passing over the wall of the housing and then the bottom of the support.
The device according to the prior art represents an improvement with respect to previously used devices, but still leaves one non negligible cause of leaking untouched and if, theorectically, it may be considered that the space between the ring and the tips of the blades is reduced to a minimum, the same is not true in actual practice. In fact, due to the porosity of the material and the pressure differences which exist between the inlet and the outlet of the turbine, a part of the air flow of the reactor passes longitudinally in the layer of porous material, thus losing some of its kinetic energy not on the blades but in the porous material of the ring, and in the process it seriously interferes with the transpiration cooling. Further, the supporting and mounting device for the ring comprises a certain number of pieces which cooperate by acting as casings. This type of mounting represents a source of leaks of the cooling air and creates thermal barriers detrimental to the favorable isothermal state of the assembly.